Respond to stimulus (5)
I believe the answer is:
react
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">respond to stimulus</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for react that I've seen before include "Hit back" , "Acknowledge" , "Change chemically" , "What a chemical might do" , "Behave in response" .)